Rennenkampf was a graduate of the Russian staff academy in St. Petersburg and was a well-known figure at the Tsarist Court. He was no stranger to active service, having served in China in 1900-1901. He commanded a cavalry division in the Russo-Japanese War of 1904-1905 and emerged from that conflict with his professional reputation enhanced.

After its catastrophic defeat by Japan, the Russian army was forced to modernize. Unfortunately, the officer corps split on the mechanics of such reform between the adherents of Sukhomlinov, the Minister of War, and those of Grand Duke Nicholas. As a result, army reforms were carried out in a muddled fashion and, particularly in such areas as logistics and heavy artillery, the army was much less prepared for what 1914 would bring than the German army was.

Rennenkampf was the commander of the Vilna Military District when the war broke out and, upon mobilization, assumed command of the First Army. The war plan called for an invasion of East Prussia by Rennenkampf's First Army and Alexander Samsonov's Second Army with the objective of forcing the Germans to withdraw troops from the invasion of France.

First Army began the invasion of East Prussia on 12 August with cavalry probes. The bulk of the army crossed into German territory five days later. Almost immediately the Russians were engaged and beaten at Stalluponen by the German I Corps under General Hermann von Francois. He had acted in disobedience to orders because he was unwilling to cede any territory to the enemy voluntarily. But von Prittwitz, the commander of the German Eighth Army compelled him to withdraw before he could exploit his victory.

As the Germans withdrew, Rennenkampf's army continued to march slowly into East Prussia. Von Prittwitz, at von Francois' urging, ordered an attack by the entire Eighth Army. The Battle of Gumbinnen (20 August) was a confused affair which resulted in a Russian tactical victory. However, Rennenkampf's supply services had all but collapsed. Artillery batteries were short of shells, the men were short of food and the horses were short of fodder. Rennenkampf sat idly while the entire Eighth Army concentrated against Samsonov's Second Army, surrounded it and destroyed it at the Battle of Tannenberg. While he was re-organizing his army. Rennenkampf made no effort to reconnoitre his front and was totally unaware that he had no significant German forces facing him.

The Battle of Tannenberg was finished on 31 August. The Germans regrouped and used the railway system of East Prussia to concentrate against Rennenkampf's army. He was defeated in the Battle of the Masurian Lakes 9-14 September) and was forced to retreat from East Prussian soil. However, his army remained intact, despite certain German claims to the contrary.

The Russian threat to East Prussia had forced the Germans to withdraw two infantry corps and a cavalry division from France, so it might be said to have accomplished its objective. But the price was high. In early 1915, the recriminations began. Rennenkampf's Baltic German heritage was noted and whispered questions about his loyalty circulated around St Petersburg. Finally, he was relieved of his command and then dismissed from the army.

In early 1918 the Bolsheviks offered him a commission in the newly-formed Red Army. Rennenkampf declined-and was shot by the Bolsheviks.